Aqueous inkjet ink as a dispersion of pigments dispersed in a water-containing dispersion medium generally contains a moisturizer that suppresses water evaporation. A viscosity adjuster that suppresses a viscosity increase caused by water evaporation is also generally contained in aqueous inkjet ink.
Polyhydric alcohol contained as the moisturizer has good compatibility to the main component cellulose of paper medium. The water-soluble organic solvent contained as the viscosity adjuster has high permeability to paper. The aqueous inkjet ink can thus easily permeate inside the paper medium, and the pigments in the ink usually do not remain on the surface of paper medium. In order to form an image of a desired density on paper medium, aqueous inkjet ink needs to contain pigments in amounts that take into account permeability for the paper medium.
However, the pigment content needs to be reduced to cut down the manufacturing cost of the ink.